1. Field of the Invention
The invention relates to a storage framework for nuclear reactor fuel elements, having a plurality of structural wells of essentially rectangular cross section fastened on a baseplate.
In order to provide for the intermediate storage of spent fuel elements from a nuclear power plant, in particular a pressurized water or boiling water reactor, the fuel elements are stored within the nuclear power plant, in particular within fuel element storage basins. It is desirable for technical and economic reasons to utilize the available storage space as efficiently as possible. In order to increase the storage capacity, storage frameworks with incorporated nuclear poison, for example with boron-containing material, are employed. The storage framework, while having sufficient mechanical stability and neutron-absorbing properties, should require as little space as possible for itself, in order to allow a high loading density for spent fuel elements.
Published European Patent Application 0 537 615 A1, corresponding to U.S. Pat. No. 5,311,563, specifies a storage framework for nuclear reactor fuel elements, in which a plurality of wells of essentially rectangular cross section are fastened on a baseplate, the wells standing vertically on the baseplate and in each case being disposed diagonally opposite one another in a checkered manner. Some of the wells located diagonally opposite one another are in each case connected to one another, along their mutually adjoining longitudinal edges with an offset, through the use of at least two connecting elements that bridge a gap formed by the offset. The pair of longitudinal edges is assigned at least one first connecting element of high rigidity, in each case in a first direction parallel to the baseplate, and at least one second connecting element of high rigidity, in each case in a second direction parallel to the baseplate. Due to the high rigidity of the connecting elements, internal transverse forces acting on the storage framework can be absorbed, without the need for an additional supporting grid in the upper region of the storage framework. It is thus also possible to load the interspaces of the storage framework which are not provided with wells, so that intermediate positions or intermediate locations for fuel elements are formed. In order to provide neutron absorption, the walls of the wells of the storage framework are composed of austenitic boron steel with a boron content of up to 2%, and the connecting elements are composed of a soft austenitic steel, the carbon content of which is lower than 0.1%. As a result, when extreme external forces act on the storage framework, the boron steel wells experience virtually no deformation, since the external forces are absorbed, where appropriate, by virtue of plastic deformations of the connecting elements. The use of an austenitic boron steel as a load-bearing structure for the wells makes those wells particularly complicated to manufacture, in order to ensure the required mechanical stability of the boron-treated steel.
Further embodiments of a storage framework for spent fuel elements, in which the respective storage framework has a neutron-absorbing material, are disclosed, for example, in U.S. Pat. No. 4,088,897, U.S. Pat. No. 4,630,738, U.S. Pat. No. 4,695,424 and U.S. Pat. No. 4,119,859. A fact common to the storage frameworks known from those publications is that the neutron-absorbing material is integrated firmly into the load-bearing structure of the storage framework. In the storage framework disclosed in U.S. Pat. No. 4,088,897, boron-containing material is disposed between an inner and an outer wall of a well for receiving a spent fuel element. In the storage framework known from U.S. Pat. No. 4,630,738, a plurality of square neutron-absorbing wells parallel to one another on a baseplate are connected firmly to the baseplate, with the neutron-absorbing material, in the form of plates made from sintered boron-treated aluminum, being fastened firmly to the respective sides of the rectangular well. The boron-treated plates are disposed between an inner and an outer well. U.S. Pat. No. 4,695,424 discloses a storage framework, in which the neutron-absorbing material, for example a boron carbide, in the form of plates is non-displaceably fastened, particularly welded, to the outside of a well for receiving spent fuel elements. U.S. Pat. No. 4,119,859 describes a storage framework for fuel elements of a nuclear power plant, with wells for receiving a fuel element in each case. There, the wells have a sandwich structure with an inner wall and an outer wall, between which a neutron-absorbing material, for example boron carbide, is intercalated.
The known measures for introducing neutron-absorbing material into a fuel element storage framework involve integrating the neutron-absorbing material firmly into the load-bearing structure of the fuel element storage basin, thus necessitating a considerable construction outlay in production terms and, where appropriate, requiring a monitoring of the neutron-absorbing material.